The object of the proposed research will be to study the general behavior and reflex role of intrathoracic cardiopulmonary receptors whose impulses travel in afferent vagal C-fibers. These vagal receptors will include deflation (J) and high-threshold inflation receptors in the lungs, epicardial receptors in the heart and "vascular" receptors in the great vessels and atriovenous junctions. Afferent impulses will be recorded from "single" vagal fibers, and we shall investigate the location, patterns of discharge and mode of stimulation of the receptors. We shall investigate the effect on receptor firing of physiological procedures (inflation of the lungs, distension of blood vessels etc.) which evoke circulatory and respiratory reflexes. We shall also examine the response of these receptors to drugs such as veratridine, phenyl diguanide and capsaicin which elicit "chemoreflex" effects. The role of the receptors in these various reflex responses will be investigated. We shall also investigate the effect of stimulating afferent vagal C-fibers on the somatic nervous system (pyramidal tract cells and monosynaptic spinal reflex).